Metal halide lamps are preferred over halogen lamps in vehicle lighting systems (e.g., automotive headlight systems) because they emit more visible light per watt and have a longer life expectancy. Metal halide lamps can also be designed to emit visible light with a frequency profile similar to sunlight which improves visibility for a given amount of light. However, unlike halogen lamps, metal halide lamps cannot be driven directly from a vehicle power supply (i.e., a vehicle's charging system) and require the use of a ballast. The ballast, strikes the lamp, and adjusts the frequency and current applied to the lamp such that the lamp emits light of the proper intensity to achieve its design life.
Electronic ballasts include a controller which controls operation of a power stage for driving the metal halide lamp. The controller can be placed in a sleep state such that the ballast does not power the lamp which allows a low power switch or electronic signal (e.g. a signal provided by a vehicle's electronic control module) to turn the lamp on and off. However, in the sleep state a relatively simple and inexpensive bias circuit which provides bias power to the controller draws a current large enough to drain the power supply of the vehicle over a relatively short period of time. For example, an electronic ballast with the controller in the sleep state can drain an automobile's battery over a weekend such that the vehicle's owner could not start the car at the beginning of the week without providing additional power to the battery. A bias circuit and controller (e.g., microcontroller) which reduce this sleep state bias power drain to an acceptable level can be designed into the electronic ballast, but are relatively complex and expensive.
Generally, there are three types of lighting control modules used in vehicles that may be used to control an electronic ballast. The first type is a relatively bulky and expensive high power switch, actuated by the vehicle operator, which provides power directly from the vehicle power supply to a lamp. The second type is a cheaper and smaller low power switch, actuated by the vehicle operator, which provides power from the power supply to an electromechanical relay. The low power switch can only provide enough power to the relay to actuate the relay; the relay provides substantially more power from the vehicle power supply to the lamp.
The third type of lighting control module is electronic. The electronic lighting control module receives user input and/or input from sensors (e.g., ambient light sensors) and other sources to determine when to light a lamp. When the electronic lighting control module determines that a lamp should be lit (e.g., the vehicle engine is running, the transmission is in drive, and there is little ambient light), it either provides power directly to the ballast or energizes an electromechanical relay which provides power from the vehicle power supply to the lamp ballast. Thus, the electronic lighting control module can be built into an existing electrical component of the vehicle such as an electronic control module. However, the electronic lighting control module must either be designed with the capacity to provide a relatively high source current required to power the ballast directly or must utilize an electromechanical relay to provide power to the ballast. Either solution (high current electronic lighting control module or the addition of an electromechanical relay), adds a cost to the vehicle lighting system.
FIG. 1 shows an example of a vehicle lighting using a vehicle lighting control module (e.g. a low power switch, a high power switch, or an electronic control) to control a relay which provides power to an electronic ballast as is known in the prior art. Referring to FIG. 1, a prior art electronic ballast 102 of a vehicle lighting system 100 provides power to a lamp 104 in response to receiving power from a relay 106. In this prior art system, a power supply 108 (i.e., a vehicle charging system) of a vehicle comprises a battery and an alternator for providing power to electrical systems of the vehicle, including the vehicle lighting system 100. In operation, an operator of the vehicle provides input to a vehicle lighting control module 110 (e.g., a headlight switch of the vehicle). Based on the operator provided input, the vehicle lighting control module 110 selectively energizes the relay 106. That is, the vehicle lighting control module 110 receives power from the vehicle power supply 108 and provides the received power to the relay 106 when the operator turns the headlight switch on. Conversely, the vehicle lighting control module 110 receives power from the vehicle power supply 108 but does not energize the relay 106 when the operator turns the headlight switch off.
When the vehicle lighting control module 110 energizes the relay 106, the relay 106 provides a supply voltage from the vehicle power supply 108 to an input filter 112 of the ballast 102. The input filter 112 filters noise from the supply voltage provided by the relay 106 and provides the filtered supply voltage to a bias regulator 114 and a power stage 116 of the ballast 102. The bias regulator 114 receives the filtered supply voltage and generates a first bias voltage for a controller 118 of the ballast 102, and a second bias voltage for the power stage 116 of the ballast 102. The controller 118 controls the power stage 116 to provide power to the lamp 104. Thus, the ballast 102 provides power to the lamp 104 in response to receiving a supply voltage from the power supply 108 via the relay 106.